From zonal containment to spot recovery: Multi-platform geostatistical reassessment of radiation risk boundaries in Fukushima (2019-2024).

This study provides a spatiotemporal reassessment of the radiological landscape within the Difficult-to-Return Zone (DTRZ) surrounding the Fukushima Daiichi Nuclear Power Plant (FDNPP), integrating airborne, car-borne, and walk-borne survey data from 2019 to 2024 through Empirical Bayesian Kriging (EBK) to generate high-resolution (50-m) dose-rate surfaces. Applying the official Japanese government occupancy threshold of 3.8 μSv·h-1 (equivalent to 20 mSv·y-1), the 2019 DTRZ boundary demonstrated complete radiological containment, with all threshold-exceeding grid cells located within the zone boundaries in both monitoring years, but exhibited pronounced internal over-restriction: 96.2% of the 337.34 km2 DTRZ area comprised grid cells that did not exceed the threshold in 2019, rising to 98.4% by 2024 as the threshold-exceeding area contracted by 56.8% from 12.74 km2 to 5.50 km2. Radiological assessment of the seven Specified Reconstruction and Revitalization Base Areas (SRRBA) (digitized from published cartography) revealed that mean dose rates across all patches declined from 6.45 mSv·y-1 to 3.08 mSv·y-1 (-52.2%), with all patches recording doses below the 20 mSv·y-1 policy threshold and threshold-exceeding area contracting by 86.1%. Forest classes (Deciduous Broadleaf Forest [DBF], Deciduous Needleleaf Forest [DNF], Evergreen Broadleaf Forest [EBF], Evergreen Needleleaf Forest [ENF], and Bamboo forest) accounted for 60.2% of threshold-exceeding cells in 2019, declining to 56.5% in 2024, with a notable compositional shift from DBF (39.6% to 25.8%) toward ENF (15.1% to 22.8%) reflecting differential radioecological recovery rates. These findings empirically validate the radiological justification for the SRRBA designation and the broader transition from broad zonal restriction toward spot-based recovery management.